A wireless power transmission or a wireless energy transfer refers to a technology for wirelessly transferring electric energy to desired devices. In the 1800's, an electric motor or a transformer employing the principle of electromagnetic induction has been extensively used and then a method for transmitting electric energy by irradiating electromagnetic waves, such as radio waves or lasers, has been suggested. Actually, electric toothbrushes or electric razors, which are frequently used in daily life, are charged based on the principle of electromagnetic induction. Until now, the long-distance transmission using the magnetic induction, the resonance and the short-wavelength radio frequency has been used as the wireless energy transfer scheme.
In the case of a short-distance wireless power transmission, which has been spotlighted in these days, a wireless power transmitter is installed in a building in such a manner that a mobile device, such as a cellular phone or a notebook computer, can be continuously charged when a user uses the mobile device in the building even if the mobile device is not connected to an additional power cable.
However, according to the wireless power transmission of the related art, power must be constantly transmitted regardless of the existence of a receiver that receives the power, causing the waste of the power.